I want to know what a Fourier transform do to a image, does magnitude of this transform represent the energy of radiations captured by image, or is there any other way to find out the energy of radiation captured by image, using image processing?
The Fourier transform represents the image using a basis of orthonormal complex sinusoids rather than the standard Dirac basis. At its core, it's nothing more than a change of basis. Its main appeal is that it separates components of the image in terms of frequency rather than space/time.
As to your other questions, you'll need to be explicit in what you mean by "energy of radiation".